Issue link: https://iconnect007.uberflip.com/i/1073397
JANUARY 2019 I FLEX007 MAGAZINE 53 location of features increasingly difficult over long distances. On a localized basis, and/or with smaller flexible circuits, the effects are not so great, and the part can more easily meet requirements. This approach does not sacrifice any tolerances. It is simply a means of recog- nizing and accounting for flex circuit manufac- turing dimensional variances. Figure 4 illus- trates the benefits of multiple datums. Marking Requirements The increased use of automation in manu- facturing minimizes the need for marking in many applications. However, some designs can still benefit from marking despite the added cost. When marking is used, the documenta- tion package must accurately define where to locate markings on the circuit. The choice of ink type and color must also be defined. Bend, Flex, and Crease Locations Flexible circuits are typically bent or flexed in some manner. Knowing where the bending is to occur is important, especially when the flex circuit is to be used in dynamic applications. In dynamic flex applications, the grain direction of the copper foil should be accounted for in manufacturing. It has been shown that there is a difference in the dynamic flex life of cir- cuits fabricated with the bend going with the machine direction and those where bends are made transverse to the machine direction. Accurate bend or flex information also informs the engineer or technician where and in which direction to bend the circuit, result- ing in easier installation. It is also beneficial to define the location of permanent bends and/or creases that may be needed, which can be accomplished in a couple of ways. If a marking step is required, special indicating features in the marking circuit artwork can be printed onto the coverlayer or while marking is applied. This facilitates assembly by providing infor- mation as to which direction a bend must take. For example, dotted lines can be used to indicate bends in one direction, and solid lines can indicate bends in the opposite direc- tion. Similar features can also be made in the metal as part of the circuit. Because there are no common standards for this task, the fab- ricator's manufacturing engineers should be able to provide some guidance based on their experiences. Stiffener Locations and Bonding Requirements Stiffeners are commonly used to support com- ponents mounted onto flexible circuits. When a design requires stiffeners, the documentation package must include size, construction, loca- tion, and the preferred material to be used to bond them to the flexible base material of the circuit. If any special strain relief techniques are required, such as an epoxy bead along flex- to-rigid transition, this should also be noted. The cross-sectional view should also show any strain relief as discussed earlier. Figure 4: Flex circuit with multiple datums.